On MacOS (Mojave) I have installed ManyCam.
I have Skype, Camfrog, Facetime, Photo booth etc.
I can select ManyCam in Skype, but it is NOT available in Camfrog, Facetime, Photo booth - they only display the Default FaceTime camera in the dropdown.

I have installed the ManyCam app on my mobile, it connects and displays in the ManyCam desktop app fine. (although my face is blue - wierd)
All I am trying to do is use my Mobile as a second webcam for my Macbook. (later maybe even an ip camera)

The reason of problem with camera visibility is the sandboxing, that Apple intentionally use in their software. There is no workaround for this problem. You may try to submit a feedback to Apple and ask to fix the issue with FaceTime, Photo booth and Safari: https://www.apple.com/feedback/macos.html The same is true for Camfrog and any other software where manycam doesn’t work.

As for the blue face problem, try to logout/login on both your devices.